1. Field of the Invention
The present invention relates to dual mode band pass filters preferably used as, for example, band filters incorporated in communication apparatuses for high frequency bands ranging from a microwave band to a millimeter-wave band.
2. Description of the Related Art
Conventional filters include dual mode band pass filters used as band pass filters in high frequency bands (See, for example, xe2x80x9cMiniature Dual Mode Microstrip Filtersxe2x80x9d, J. A. Curtis and S. J. Fiedziuszko, 1991 IEEE MTT-S Digest, etc.)
FIGS. 13 and 14 show schematic plan views for illustrating conventional dual mode band pass filters.
In a band pass filter 200 shown in FIG. 13, a circular conductive film 201 is disposed on a dielectric body (not shown). The conductive film 201 is coupled to input/output coupling circuits 202 and 203 arranged to define an angle of 90 degrees. A top-end open stub 204 is arranged to form a central angle of 45 degrees with the position where the input/output coupling circuit 203 is arranged. With this arrangement, two resonance modes having different resonance frequencies are mutually coupled. As a result, the band pass filter 200 functions as a dual mode band pass filter. In addition, in a dual mode band pass filter 210 shown in FIG. 14, a square conductive film 211 is disposed on a dielectric body. The conductive film 211 is coupled to input/output coupling circuits 212 and 213 defining an angle of about 90 degrees. A corner defining an angle of about 135 degrees with the input/output coupling circuit 213 is cut away. By disposing a cut-away portion 211a, two resonance modes have different resonance frequencies. With this arrangement, since the two resonance modes are mutually coupled, the band pass filter 210 functions as a dual mode band pass filter.
On the other hand, as an alternative to a circular conductive film, a ring-shaped conductive film is used in dual mode band pass filters (Japanese Unexamined Patent Application Publication No. 9-139612, Japanese Unexamined Patent Application Publication No. 9-162610, etc.). In this case, with the use of a ring-shaped transmission line, as in the case of the dual mode band pass filter shown in FIG. 13, input/output coupling circuits are arranged at a central angle of 90 degrees, and a top-end open stub is disposed in a portion of the ring-shaped transmission line.
Furthermore, Japanese Unexamined Patent Application Publication No. 6-112701 provides a dual mode band pass filter using a similar ring-shaped transmission line. As shown in FIG. 15, a dual mode filter 221 includes a ring-shaped resonator defined by disposing a ring-shaped conductive film 222 on a dielectric body. In this case, each of the four terminals 223 to 226 is arranged to define an angle of 90 degrees with the ring-shaped conductive film 222. Of the four terminals, the two terminals 223 and 224 defining an angle of 90 degrees are coupled to input/output coupling circuits 227 and 228. The remaining two terminals 225 and 226 are connected to each other via a feedback circuit 230.
With this arrangement, in the ring-shaped resonator defined by one stripline, there are generated vertical resonance modes that are not coupled to each other. As a result, it is possible to control the coupling strength via the feedback circuit 230.
In each of the conventional dual mode band pass filters shown in FIGS. 13 and 14, with the use of one conductive film pattern, a two-stage band pass filter can be provided. As a result, miniaturization of the band pass filter can be achieved.
However, in such a circular or square conductive film pattern, since the input/output coupling circuits are coupled at the predetermined angle, the coupling strength cannot be increased. Thus, there is a problem in that a wider pass band cannot be obtained.
In the band pass filter shown in FIG. 13, the conductive film 201 has a circular shape. In the band pass filter shown in FIG. 14, the conductive film 211 has a square shape. Thus, the shapes of the conductive films are restricted. As a result, there is little freedom of design.
Furthermore, similarly, it is difficult to increase the coupling strength and there are restrictions on the shapes of the ring-shaped resonators in the dual mode band pass filters using the ring-shaped resonators in Japanese Unexamined Patent Application Publication No. 9-139612 and Japanese Unexamined Patent Application Publication No. 9-162610, as mentioned above.
On the other hand, in the dual mode band pass filter 221 described in Japanese Unexamined Patent Application Publication No. 6-112701, coupling strength is adjusted by using the feedback circuit 230 so that a wider bandwidth is obtained. However, since this dual mode filter needs the feedback circuit 230, the circuit structure is complicated. In addition, still, the shape of the resonator is restricted to a ring shape, thereby reducing the freedom of design.
In order to overcome the problems described above, preferred embodiments of the present invention provide a dual mode band pass filter that achieves miniaturization, facilitates adjustments of the coupling strength, achieves a wider pass band and greatly improves the freedom of design.
According to a first preferred embodiment of the present invention, there is provided a dual mode band pass filter including a dielectric body having a first main surface and a second main surface, a metal film partially disposed on the first main surface or at a certain height position in the dielectric body, at least one ground electrode disposed on the second main surface or inside the dielectric body in such a manner that the metal film is opposed to the ground electrode via a portion of the dielectric body, and a pair of input/output coupling circuits coupled to different parts of the metal film. In this dual mode band pass filter, in the region where the metal film is opposed to the ground electrode via the portion of the dielectric body, some portions of the dielectric body have relative permittivities that are different from a relative permittivity of the remaining portion so that two resonance modes generated at the metal film are mutually coupled.
In the dual mode band pass filter according to the first preferred embodiment of the present invention, the two resonance modes are generated in a direction that is substantially parallel to a virtual line connecting the portions coupling the pair of input/output coupling circuits to the metal film and in a direction that is substantially to the virtual line. In addition, in order to couple the two resonance modes, relative permittivities of the portions of the dielectric body in the region where the metal film is opposed to the ground electrode via the dielectric body are made different from the relative permittivity of the remaining portion. In other words, one of the two resonance modes is influenced by the dielectric-body portions having the different relative permittivities, and the resonance frequency of the influenced resonance mode thereby changes. As a result, the two resonance modes are mutually coupled. That is, since the portions of the dielectric body have different relative permittivities from that of the remaining portion, the band pass filter functions as a dual mode band pass filter.
In addition, the portions of the dielectric body having the different relative permittivities may be cavities formed in the dielectric body.
According to a second preferred of the present invention, a dual mode band pass filter includes a dielectric body having a first main surface and a second main surface, a metal film partially disposed on the first main surface or at a certain height position of the dielectric body, at least one ground electrode disposed on the second main surface or inside the dielectric body in such a manner that the metal film is opposed to the ground electrode via a portion of the dielectric body, and a pair of input/output coupling circuits coupled to different portions of the metal film. In this dual mode band pass filter, openings or cut-away portions are provided in the ground electrode in the region where the metal film is opposed to the ground electrode so that two resonance modes generated at the metal film are mutually coupled.
In this dual mode band pass filter, in order to couple the two resonance modes, in the region where the metal film is opposed to the ground electrode, the openings or the cut-away portions are provided in the ground electrode. As a result, two resonance modes are generated so as to propagate in a direction substantially parallel to a visual line connecting the portions for coupling the pair of input/output coupling circuits to the metal film and in a direction that is substantially perpendicular to the virtual line. One of the two resonance modes is influenced by the openings or the cut-away portions, with the result that the resonance frequency of the mode changes. In other words, the openings or the cut-away portions are arranged such that the openings or the cut-away portions influence the resonance electric fields or resonance currents of one of the resonance modes so as to mutually couple the two resonance modes. As a result, since the two resonance modes are mutually coupled by the openings or the cut-away portions, the band pass filter functions as a dual mode band pass filter.
Furthermore, the metal film may be disposed on the first main surface of the dielectric body and the ground electrode may be disposed on the second main surface thereof.
In addition, the shape of the metal film may have lengthwise directions and widthwise directions.
In addition, the planar shape of the metal film may be any of substantially rectangular, substantially rhombic, regular polygonal, substantially circular, or substantially elliptical.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the present invention.